A method to measure resistivity, mobility, and absorber thickness in thin-film solar cells with application to CdTe devices

We report a method developed upon coordinated admittance spectroscopy and capacitance-voltage techniques to measure resistivity, mobility, and absorber thickness in thin-film photovoltaic devices. The absorber thickness is measured by depletion region width at freeze-out temperatures when the free carriers cease to respond to bias modulation. Based on a lumped-parameter equivalent-circuit model, we derive the inflection frequency due to dielectric relaxation of the absorber. We show that the square of freeze-out frequency depends linearly on bias voltage. Resistivity-and mobility-is calculated from the slope of this linear dependence. To demonstrate this method, we applied it to thin-film CdTe solar cells with back contacts formed under three different conditions: (A) with Cu in the carbon paste after nitric-phosphoric etch, (B) without Cu in the carbon paste after nitric-phosphoric etch, and (C) without Cu in the carbon paste and without nitric-phosphoric etch. The measured absorber thicknesses (5.45, 5.85, and 7.95 mu m, respectively) agree well with growth history and other methods. Study using this method also yields insights to back-contact formation mechanism in terms of etching loss, Te-rich layer, and Cu doping/alloying. The freeze-out exhibits thermal activation due to combined contribution from mobility and carrier concentration. (C) 2010 Elsevier B.V. All rights reserved.